The present invention relates generally to pressure-sensitive touch pads for data entry and, more particularly, to a self-calibrating system employing low-cost transducer elements, such as variable-capacitance touch pads, subject to parameter drift.
So-called touch pads are employed for data entry in a wide variety of equipment, such as consumer appliances including microprocessor- or microcontroller-based control systems. Typically, such touch pads include either physical switch contacts often comprising a snap dome for tactile feedback, or an on/off capacitive touch switch wherein the proximity or touch of a finger on a stationary touch pad is sensed by a change in signal level, such as by shunting a signal to ground, whereby the touch is recognized. In the case of such a capacitive touch switch, the change is recognized when the signal level passes a predetermined threshold.
Often such touch pads are used to enter data into an appliance or instrument where the data is represented on a digital display, for example, a digital clock, a digital timer, or a digital indicator of cooking temperature. For setting such digital displays and the underlying function value, up and down slew pads are typically included and arranged such that one pad causes the indicated value to count up, while the other causes the indicated value to count down.
In many cases, it is desirable to provide different or variable slew rates such that the indicated value changes rapidly where the numerical difference between a present and a target reading is great, and changes slowly when the difference between the indicated and the target reading is small. In such cases, two or more slew pads for each direction may be provided, for selecting variable slew rates. A disadvantage of this approach is that additional control panel area is required.
Various forms of variable touch pad data entry systems have been proposed. For example, Bobick U.S. Pat. No. 4,103,252 and Meno U.S. Pat. No. 4,694,279 disclose systems employing position-sensitive capacitive touch switch devices. These systems, however, differ little in concept from systems employing a plurality of discrete pads for providing different slew rates as briefly described above, and share the disadvantage that a larger-than-necessary control panel area is required.
Another approach is represented by Kataoka U.S. Pat. No. 4,673,919, which discloses a multi-point variable pressure sensitive touch key device which provides a plurality of different output pulse frequencies for driving a counter, depending upon the touch pressure. In the Kataoka system, the pressure transducer device comprises a potentiometer.
A disadvantage to such an approach, particularly in a consumer appliance, relates to the accuracy and repeatability of the transducer, particularly where multi-point operation is desired from a single touch pad. In the case of a capacitive touch pad, for example, capacitance will tend to change with changes in temperature, flatness of any overlay, and humidity. In addition, manufacturing and assembly variations can result in each pad having a slightly different capacitance when manufactured, potentially requiring calibration procedures. Nevertheless, capacitive touch pads have desirable characteristics including high reliability resulting from the absence of electrical switch contacts, and ease in providing a flat control panel.
Summarizing the foregoing, attempts to provide touch key systems in general, and multi-point variable-pressure touch key systems in particular, are complicated by parameter variations in the pressure transducers employed, whether resistive or capacitive. Providing a more expensive transducer is not always an acceptable solution, particularly where the advantages of capacitive touch pads are desired. Multi-point variable-pressure touch key systems are advantageous in many situations because they minimize the amount of control panel area required, and can result in simpler user operation.